1. Field of the Invention
The present invention relates to a phase shifter for use with a receiving unit for receiving a radio frequency signal such as a radio communication signal and for demodulating the received signal.
2. Description of the Related Art
A radio receiving unit such as a portable telephone set has a quadrature demodulating circuit and an image eliminating frequency converter. In the quadrature demodulator and the image eliminating frequency converter, a multiplying circuit and a phase shifter are used as major structural elements.
Hiroyuki Kikuchi et. al. has disclosed a phase shifter in "GHz-Band Monolithic Modem IC's", IEEE TRANSACTION ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTT-35, NO. 12 DECEMBER 1987 (FIG. 5).
In this phase shifter, the phase is adjusted with an external control voltage. This phase shifter was provided to prevent a fabrication error that takes place in a phase shifter formed on an integrated circuit.
Kazuya Yamamoto et. al. has disclosed a phase shifter in "A 1.9-GHz-Band GaAs Direct-Quadrature Modulator IC with a Phase Shifter", IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL 28, NO. 10, OCTOBER 1993, PP 994-1000 (FIG. 6).
This phase shifter has a first filter circuit and a second filter circuit. The first filter circuit has a differential circuit with resistor elements and capacitor elements for a high pass characteristic. The second filter circuit has an integrating circuit with resistor elements and capacitor elements for a low pass characteristic. Thus, the phase sifter output two signals whose phases deviate from each other by 90.degree.. In this circuit, when the relative accuracies of these elements are high, two output signals with small phase errors can be obtained.
However, in a portable telephone set, the phase shifter is used as a part of the RF stage. Thus, the phase shifter should be structured as one chip (namely, the phase shifter should be composed of an IC chip).
However, since the absolute accuracies of the elements on the IC chip cannot be improved, if the element structures of the individual filter circuits are changed as with the phase shifter of Kazuya Yamamoto et. al, it is difficult to match the amplitudes of two signals obtained from these filter circuits.
On the other hand, Jan Crols has disclosed a phase shifter in "A Single Chip 900 MHZ COMS Receiver Front-end with a High Performance Low-IF Topology", IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 30, NO. 12, DECEMBER 1995, PP 1483-1492 (FIG. 6). In this phase shifter, many resistor elements and many capacitor elements are used so as to reduce errors of phases and errors of amplitudes.
However, in this case, output signals should have phases that deviate by 90.degree. to the phase of the input signals.
Thus, output signals with phases that deviate from the phase of an input signal by for 45.degree. cannot be obtained.
A phase shifter that generates output signals with phases that deviate by 45.degree. has been disclosed in U.S. Pat. No. 4,612,518. This phase shifter is used for a vector modulating circuit with a 1/8 .lambda. transmission line and a harmonic mixer.
In this case, since a group delay on the transmission line does not depend on a frequency for use, the phase shift amount can be varied in proportion to the frequency. However, to vary the phase shift amount, the length of the transmission line should be varied. Thus, the length of the transmission line should be long in radio frequencies such as microwave and GHz band used for portable telephone sets, radio units, and so forth. Consequently, such a transmission line cannot be physically formed on an IC chip.